Update leaderboard names to show ranking_score instead of validation_score

[christopherbunn]

Change validation_score to ranking_score. This PR will also show the holdout_score if there is a holdout set in the leaderboard.

[codecov]

Codecov Report

Merging #3878 (25800bb) into main (9f08237) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3878     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        346     346             
  Lines      36325   36352     +27     
=======================================
+ Hits       36188   36221     +33     
+ Misses       137     131      -6     

Impacted Files	Coverage Δ
...ts/automl_tests/parallel_tests/test_automl_dask.py	96.1% <ø> (ø)
evalml/automl/automl_search.py	99.6% <100.0%> (+0.2%)	⬆️
evalml/automl/pipeline_search_plots.py	100.0% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (+0.1%)	⬆️
...ts/automl_tests/test_automl_iterative_algorithm.py	100.0% <100.0%> (ø)
...l/tests/automl_tests/test_pipeline_search_plots.py	100.0% <100.0%> (ø)
.../integration_tests/test_time_series_integration.py	100.0% <100.0%> (ø)
evalml/automl/engine/engine_base.py	100.0% <0.0%> (+3.4%)	⬆️

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[eccabay]

LGTM!

[jeremyliweishih]

LGTM as well